1. Field of the Invention
The present invention relates to a surface texture measurement device, a controller for the surface texture measurement device, and a method for controlling the surface texture measurement device. In particular, the present invention relates to a surface texture measurement device having a plurality of display ranges and to a method for controlling the surface texture measurement device.
2. Description of Related Art
Conventionally, a surface texture measurement device is known in which a surface of a work piece is scanned by a stylus to measure a surface texture thereof (surface roughness, undulations, shape in outline, and the like). In the surface texture measurement device, the stylus is moved in a fixed direction (X-axis direction) and is displaced in a vertical direction (Z-axis direction) of the stylus due to unevenness in the surface of the work piece. Amplification or A/D conversion of a detection signal is performed, then the signal is displayed on a display as a function of movement distance. In addition, in a roundness measurement device, which is a type of surface texture measurement device, the stylus is in stationary contact with an outer peripheral surface of a work piece having a rotational form. By then rotating the work piece, the outline shape is detected for one rotation of the work piece.
A displacement sensor used in the surface texture measurement device has a high level of sensitivity in general; however, a detection stroke (measurable range) of the displacement sensor is not large. In addition, a detection resolution is limited due to a performance of an A/D converter or due to a noise level of an amplifier, and thus the detection resolution cannot be made indefinitely larger. Thus, an amplification factor of the amplifier is switched to a plurality of levels and an appropriate selection can be made for display from a range with a high resolution and a short stroke through a range with a low resolution but a long stroke.
In a surface texture measurement device 90 in FIG. 7, when a driver 92 moves an arm 93 in the X-axis direction with a command from a controller 91, a stylus 94 displaces in the Z-axis direction according to unevenness in a surface of a work piece 95. The displacement is detected by a displacement sensor 96 and is sent to the controller 91. In the controller 91, a detection signal from the displacement sensor 96 is amplified by a range amplifier 97, and is then displayed on a display 98 as a graph of displacement amount corresponding to a scan position. For example, even in a case where the unevenness is unclear when the range amplifier 97 is set to a 1× range, as in a display 98A, by setting the range amplifier 97 to a 10× range, the unevenness can be clearly identified, as in a display 98B.
As a surface texture measurement device having a plurality of display ranges as described above, Japanese Patent Laid-open Publication Nos. 2000-310529 and H05-34145 are known. In Japanese Patent Laid-open Publication No. 2000-310529, displays in a plurality of display ranges are automatically switched in response to measurement data, thus improving appropriateness and efficiency of a measurement operation. In Japanese Patent Laid-open Publication No. H05-34145, for displays in a plurality of display ranges, an offset amount for each range can be automatically controlled for measured data, thus improving appropriateness and efficiency of a measurement operation.
In a surface texture measurement device, when switching between a plurality of display ranges to perform display, display errors may arise due to characteristics of a processing system for each display range or the like. Specifically, in each of the display ranges of the surface texture measurement device, one measurement value is converted to a display value with gain for each range. However, when there is an error in an amplifier for each range, there is a possibility that even when the measurement value is the same, a different display value will result for each display range. For example, at a 10 μm range, a display is 0.60 μm; however, at a 1 μm range, a display is 0.61 μm.
In response to such errors between ranges, a user can resolve the errors by controlling a device for each display range. Meanwhile, when switching between display ranges is performed automatically as in Japanese Patent Laid-open Publication No. 2000-310529, when a user controls each of the switches, the benefit of automation is undermined. Thus, a technology capable of automating even error control, as in Japanese Patent Laid-open Publication No. H05-34145, is very meaningful. The errors between ranges that Japanese Patent Laid-open Publication No. H05-34145 attempts to resolve are chiefly errors in an offset amount for each range. Offset errors are representative of errors between ranges; however, it has become clear that simply resolving offset errors is insufficient for resolving errors between ranges.
In FIG. 8, the display values are shown on the vertical axis with respect to the measurement values on the horizontal axis. The relationship between each value is basically a proportional relation having a positive slope. In conventional switching between display ranges, when the measurement value is small, a display range R1 having a high amplification factor is used. As the measurement value becomes larger, display is performed by switching to display ranges R2 and R3. Errors between ranges arise in each of the display ranges R1 to R3. As mentioned previously, the offset error is representative of the error between ranges and appears in the graph of FIG. 8 as a step between each display range. The offset error corresponds to parallel translation on the graph; however, actual errors between ranges also appear as an inclination in the graph. Accordingly, simply resolving the offset error described above stalls at a partial resolution of the error between ranges. Thus, development of a technology capable of resolving errors for each entire display range is desired.